Most conventional television and other display devices are built around CRTs (cathode ray tubes). The CRT is however being replaced by liquid crystal displays, particularly those of matrix types, due to the latter's recent improvements in viewing angle, contrast, and color reproduction, as well as for their small dimensions and power saving features.
A matrix liquid crystal display includes: a set of scan signal lines in the display area; a scan signal line drive circuit (gate driver) supplying the scan signal lines with scan signals; a set of data signal lines positioned to cross the set of scan signal lines at right angles; a data signal line drive circuit (data driver) supplying data signals to the data signal lines in accordance with display signals; a control circuit (controller) controlling the scan signal line drive circuit and the data signal line drive circuit; and switching TFTs (thin film transistors) located where the scan signal lines cross the data signal lines for pixel controlling purposes. The data signals are applied to pixel electrodes connected to those TFTs selected by the scan signals, to control the alignment of liquid crystal in the pixels by means of the potential difference from the opposite electrode.
Liquid crystal provides a capacitive load and is therefore, when a data signal voltage is applied to the pixel electrode, aligned and held in that state in accordance with the applied data signal voltage. Thus, liquid crystal is said to have a “hold” property. The liquid crystal produces a flicker-free display when compared to the CRT. The liquid crystal, however, develops afterimages and other degradation in quality in movies due to its slow response speed. The response speed is especially slow when effecting grayscale, because the liquid crystal does not respond sufficiently in one frame period of a video input signal.
The liquid crystal has other problems too. When a TFT is deselected, the data signal written to a corresponding pixel is held. Therefore, for example, even with a liquid crystal with improved response speed, afterimages persist on the retina because the observer's eye tracks the moving image.
Addressing these problems is, among others, Japanese unexamined patent application 11-109921/1999 (Tokukaihei 11-109921; published on Apr. 23, 1999) disclosing s liquid crystal display method. A US patent is granted on an equivalent to the Japanese application (U.S. Pat. No. 6,396,469 issued on May 28, 2002).
According to the liquid crystal display method disclosed in the Japanese application, the screen is horizontally divided into two portions: a pixel array 101a and a pixel array 101b (see FIG. 12). In the first half of a frame period, the upper portion of the screen is scanned with data signals, and the lower portion of the screen is scanned with black (blank) signals at the same time. In the latter half of the frame period, the upper portion of the screen is scanned with black (blank) signals, and the lower portion of the screen is scanned with data signals.
According to the liquid crystal display method, each pixel goes through both an image display period and a black display period in a frame period. See FIG. 13. The very presence of the black display period enables an image display with two successive frame data sets being clearly separated. The separation improves display performance in relation to afterimages in movies.
However, this conventional liquid crystal display method suffers from poor brightness across the display screen, because either the upper or lower portion of the screen is always producing a black display.
Specifically, each frame period is divided into the first and second halves. The screen is also divided horizontally into two portions. Further, in the first half of the frame period, the upper portion of the screen is scanned with data signals, and the lower portion of the screen is scanned with black (blank) signals at the same time. In the latter half of the frame period, the upper portion of the screen is scanned with black (blank) signals, and the lower portion of the screen is scanned with data signals.